Segmented side-to-side FAIMS

ABSTRACT

Provided is a side-to-side high field asymmetric waveform ion mobility spectrometer (FAIMS) including a generally cylindrically-shaped inner electrode having a length. Encircling the inner electrode is a generally cylindrically-shaped outer electrode assembly comprising at least first and second and third outer electrode segments. Each of the outer electrode segments has a channel extending therethrough and open at opposite ends thereof. In an assembled condition, the second outer electrode segment is disposed intermediate the first and third outer electrode segments in an end-to-end arrangement, each one of the first and second and third electrode segments overlapping a different portion of the length of the inner electrode. An electrical contact is provided on at least one of the inner electrode and the second outer electrode segment for receiving a first direct current voltage between the inner electrode and the second outer electrode segment, and for applying an asymmetric waveform voltage to the at least one of the inner electrode and the second outer electrode segment. The second outer electrode segment is maintained at a different potential relative to the first and third outer electrode segments, such that a potential gradient is formed in a direction along the length of the inner electrode.

This application claims the benefit of U.S. Provisional Application No.60/354,711 filed Feb. 8, 2002.

FIELD OF THE INVENTION

The instant invention relates generally to high field asymmetricwaveform ion mobility spectrometry (FAIMS), more particularly theinstant invention relates to a side-to-side FAIMS having a generallycylindrical inner electrode and a multi-segmented generally cylindricalouter electrode assembly.

BACKGROUND OF THE INVENTION

High sensitivity and amenability to miniaturization for field-portableapplications have helped to make ion mobility spectrometry (IMS) animportant technique for the detection of many compounds, includingnarcotics, explosives, and chemical warfare agents as described, forexample, by G. Eiceman and Z. Karpas in their book entitled “IonMobility Spectrometry” (CRC, Boca Raton, 1994). In IMS, gas-phase ionmobilities are determined using a drift tube with a constant electricfield. Ions are separated in the drift tube on the basis of differencesin their drift velocities. At low electric field strength, for example200 V/cm, the drift velocity of an ion is proportional to the appliedelectric field strength, and the mobility, K, which is determined fromexperimentation, is independent of the applied electric field.Additionally, in IMS the ions travel through a bath gas that is atsufficiently high pressure that the ions rapidly reach constant velocitywhen driven by the force of an electric field that is constant both intime and location. This is to be clearly distinguished from thosetechniques, most of which are related to mass spectrometry, in which thegas pressure is sufficiently low that, if under the influence of aconstant electric field, the ions continue to accelerate.

E. A. Mason and E. W. McDaniel in their book entitled “TransportProperties of Ions in Gases” (Wiley, New York, 1988) teach that at highelectric field strength, for instance fields stronger than approximately5,000 V/cm, the ion drift velocity is no longer directly proportional tothe applied electric field, and K is better represented by K_(H), anon-constant high field mobility term. The dependence of K_(H) on theapplied electric field has been the basis for the development of highfield asymmetric waveform ion mobility spectrometry (FAIMS). Ions areseparated in FAIMS on the basis of a difference in the mobility of anion at high field strength, K_(H), relative to the mobility of the ionat low field strength, K. In other words, the ions are separated due tothe compound dependent behavior of K_(H) as a function of the appliedelectric field strength.

In general, a device for separating ions according to the FAIMSprinciple, has an analyzer region that is defined by a space betweenfirst and second spaced-apart electrodes. The first electrode ismaintained at a selected dc voltage, often at ground potential, whilethe second electrode has an asymmetric waveform V(t) applied to it. Theasymmetric waveform V(t) is composed of a repeating pattern including ahigh voltage component, V_(H), lasting for a short period of time t_(H)and a lower voltage component, V_(L), of opposite polarity, lasting alonger period of time t_(L). The waveform is synthesized such that theintegrated voltage-time product, and thus the field-time product,applied to the second electrode during each complete cycle of thewaveform is zero, for instance V_(H)t_(H)+V_(L)t_(L)=0; for example+2000 V for 10 μs followed by −1000 V for 20 μs. The peak voltage duringthe shorter, high voltage portion of the waveform is called the“dispersion voltage” or DV, which is identically referred to as theapplied asymmetric waveform voltage.

Generally, the ions that are to be separated are entrained in a streamof gas flowing through the FAIMS analyzer region, for example between apair of horizontally oriented, spaced-apart electrodes. Accordingly, thenet motion of an ion within the analyzer region is the sum of ahorizontal x-axis component due to the stream of gas and a transversey-axis component due to the applied electric field. During the highvoltage portion of the waveform an ion moves with a y-axis velocitycomponent given by v_(H)=K_(H)E_(H), where E_(H) is the applied field,and K_(H) is the high field ion mobility under operating electric field,pressure and temperature conditions. The distance traveled by the ionduring the high voltage portion of the waveform is given byd_(H)=v_(H)t_(H)=K_(H)E_(H)t_(H), where t_(H) is the time period of theapplied high voltage. During the longer duration, opposite polarity, lowvoltage portion of the asymmetric waveform, the y-axis velocitycomponent of the ion is v_(L)=KE_(L), where K is the low field ionmobility under operating pressure and temperature conditions. Thedistance traveled is d_(L)=v_(L)t_(L)=KE_(L)t_(L). Since the asymmetricwaveform ensures that (V_(H)t_(H))+(V_(L)t_(L))=0, the field-timeproducts E_(H)t_(H) and E_(L)t_(L) are equal in magnitude. Thus, ifK_(H) and K are identical, d_(H) and d_(L) are equal, and the ion isreturned to its original position along the y-axis during the negativecycle of the waveform. If at E_(H) the mobility K_(H)>K, the ionexperiences a net displacement from its original position relative tothe y-axis. For example, if a positive ion travels farther during thepositive portion of the waveform, for instance d_(H)>d_(L), then the ionmigrates away from the second electrode and eventually will beneutralized at the first electrode.

In order to reverse the transverse drift of the positive ion in theabove example, a constant negative dc voltage is applied to the secondelectrode. The difference between the dc voltage that is applied to thefirst electrode and the dc voltage that is applied to the secondelectrode is called the “compensation voltage” (CV). The CV voltageprevents the ion from migrating toward either the second or the firstelectrode. If ions derived from two compounds respond differently to theapplied high strength electric fields, the ratio of K_(H) to K may bedifferent for each compound. Consequently, the magnitude of the CV thatis necessary to prevent the drift of the ion toward either electrode isalso different for each compound. Thus, when a mixture including severalspecies of ions, each with a unique K_(H)/K ratio, is being analyzed byFAIMS, only one species of ion is selectively transmitted to a detectorfor a given combination of CV and DV. In one type of FAIMS experiment,the applied CV is scanned with time, for instance the CV is slowlyramped or optionally the CV is stepped from one voltage to a nextvoltage, and a resulting intensity of transmitted ions is measured. Inthis way a CV spectrum showing the total ion current as a function ofCV, is obtained.

Guevremont et al. have described the use of curved electrode bodies, forinstance inner and outer cylindrical electrodes, for producing atwo-dimensional atmospheric pressure ion focusing effect that results inhigher ion transmission efficiencies than can be obtained using, forexample, a FAIMS device having parallel plate electrodes. In particular,with the application of an appropriate combination of DV and CV an ionof interest is focused into a band-like region in the annular gapbetween the cylindrical electrodes as a result of the electric fieldswhich change with radial distance. Focusing the ions of interest has theeffect of reducing the number of ions of interest that are lost as aresult of the ion suffering a collision with one of the inner and outerelectrodes. FAIMS devices with cylindrical electrode geometry have beendescribed in the prior art, as for example in U.S. Pat. No. 5,420,424,the contents of which are incorporated herein by reference.

In WO 00/08455, the contents of which are incorporated herein byreference, Guevremont and Purves describe a domed-FAIMS analyzer. Inparticular, the domed-FAIMS analyzer includes a cylindrical innerelectrode having a curved surface terminus proximate an ion outletorifice of the FAIMS analyzer region. The curved surface terminus issubstantially continuous with the cylindrical shape of the innerelectrode and is aligned co-axially with the ion outlet orifice. Duringuse, the application of an asymmetric waveform to the inner electroderesults in the normal ion-focusing behavior as described above, and inaddition the ion-focusing action extends around the generallyspherically shaped terminus of the inner electrode. This causes theselectively transmitted ions to be directed generally radially inwardlywithin the region that is proximate the terminus of the inner electrode.Several contradictory forces are acting on the ions in this region nearthe terminus of the inner electrode. The force of the carrier gas flowtends to influence the ions to travel towards the ion-outlet orifice,which advantageously also prevents the ions from migrating in a reversedirection, back towards the ion source. Additionally, the ions that gettoo close to the inner electrode are pushed back away from the innerelectrode, and those near the outer electrode migrate back towards theinner electrode, due to the focusing action of the applied electricfields. When all forces acting upon the ions are balanced, the ions areeffectively captured in every direction, either by forces of the flowinggas, or by the focusing effect of the electric fields of the FAIMSmechanism. This is an example of a three-dimensional atmosphericpressure ion trap, as described in greater detail by Guevremont andPurves in WO 00/08457, the contents of which are incorporated herein byreference.

Guevremont and Purves further disclose a near-trapping mode of operationfor the above-mentioned domed-FAIMS analyzer, which achieves iontransmission from the domed-PAIMS to a mass spectrometer with highefficiency. Under near-trapping conditions, the ions that accumulate inthe three-dimensional region of space near the spherical terminus of theinner electrode are caused to leak from this region, being pulled by aflow of gas towards the ion-outlet orifice. The ions that are extractedfrom this region do so as a narrow, approximately collimated beam, whichis pulled by the gas flow through the ion-outlet orifice and into asmaller orifice leading into the vacuum system of the mass spectrometer.Accordingly, a tandem domed-FAIMS/MS device is a highly sensitiveinstrument that is capable of detecting and identifying ions of interestat part-per-billion levels.

More recently, in WO 01/69216 the contents of which is incorporatedherein by reference, Guevremont and Purves describe a so-called“perpendicular-gas-flow-FAIMS”, which is identically referred to as aside-to-side FAIMS. The analyzer region of the side-to-side FAIMS isdefined by an annular space between inner and outer cylindricalelectrodes. In particular, ions that are introduced into the analyzerregion of the side-to-side FAIMS are selectively transmitted in adirection that is generally around the circumference of the innerelectrode. For instance, the ion inlet and the ion outlet of aside-to-side FAIMS device are disposed, one opposing the other, within asurface of the outer electrode such that ions are selectivelytransmitted through the curved analyzer region between the ion inlet andthe ion outlet along a continuously curving ion flow path absent aportion having a substantially linear component. In particular, the ionstravel from the ion inlet to the ion outlet by flowing around the innerelectrode in one of a “clock-wise” and a “counter clock-wise” direction.This is in contrast to the above-mentioned FAIMS devices in which theions are selectively transmitted along the length of the innerelectrode.

Advantageously, the side-to-side FAIMS device reduces the minimumdistance that must be traveled by the ions within the analyzer region toapproximately fifty percent of the circumference of the inner electrode.Since the ions split into two streams traveling in opposite directionsaround the inner electrode after they are introduced through the ioninlet, the effective ion density within the analyzer region is reduced,and so too is the ion-ion repulsion space charge effect reduced.Furthermore, the reduction of the minimum ion travel distance has theadded benefit of improving the ion transmission efficiency. For example,by keeping the time for travel short, the effect of diffusion andion-ion repulsion forces are minimized. In keeping distances short, thetransit time of the ions through the analyzer region is also short,which supports more rapid analysis of ion mixtures.

Of course, the side-to-side FAIMS device also has some limitations. Forexample, ion separation occurs only within a very small portion of theanalyzer region of a side-to-side FAIMS. With only two possible ion flowdirections through the analyzer region, the ion concentration at a pointalong either ion flow direction remains relatively high. As the ionstransit the analyzer region, diffusion and ion-ion repulsion forces,even though they are small, cause the ions to spread out in a directionalong the length of the inner and outer electrodes. Accordingly, theions are introduced through the ion inlet as an approximately collimatedbeam of ions, but rapidly spread out to form a sheet of ions thattravels around the inner electrode to the ion outlet. Furthermore, ionsare focused between the inner and outer electrodes as a result of theapplication of the applied CV and DV, but this focusing occurs only in adirection that is approximately normal to the electrode surfaces, i.e.in a radial direction. As such, there is no force capable of focusingthe ions in a direction that is parallel to the electrode surfaces, i.e.in a longitudinal direction. Since the ions spread out slightly duringseparation, some of the ions become entrained in portions of theanalyzer region where the gas flow rate is low or stagnant. Consequentlythe ion transmission efficiency from the FAIMS to, for example, anexternal mass spectrometer is reduced.

Additionally, the strength of the focusing field between the inner andouter electrodes is related to the radius of the cylindrically shapedinner electrode. In order to produce stronger focusing fields, it isnecessary to utilize an inner electrode with a smaller radius. Ofcourse, a FAIMS analyzer having a smaller inner electrode also has asmaller available volume for separating ions. The distance between theion inlet orifice and the ion outlet orifice is also smaller, and mayresult in insufficient ion transit times to effect separation of amixture that contains different ionic species having similar high fieldion mobility properties.

It would be advantageous to provide a FAIMS apparatus including adetection system that overcomes the limitations of the prior art.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided an apparatus forseparating ions, comprising: an inner electrode assembly comprising atleast an inner electrode segment, the at least an inner electrodesegment having a length and a curved outer surface in a directiontransverse to its length and an outer electrode assembly comprising atleast an outer electrode segment, the at least an outer electrodesegment having a length, a channel extending through at least a portionthereof, and a curved inner surface, a portion of the length of the atleast an outer electrode segment overlapping a portion of the length ofthe at least an inner electrode segment and forming an analyzer regiontherebetween, the at least an outer electrode segment beingapproximately coaxially aligned with the at least an inner electrodesegment, at least one of the inner and outer electrode assembliescomprising a segmented electrode assembly having at least first andsecond and third segmented electrode segments and a length, disposedadjacent another electrode having a length, the second segmentedelectrode segment disposed intermediate the first and third segmentedelectrode segments in an end-to-end arrangement, a surface of thesegmented electrode disposed opposite to a surface of the otherelectrode; and, at least a first contact on at least one of the otherelectrode and the second segmented electrode segment for receiving afirst direct current voltage between the other electrode and the secondsegmented electrode segment, and for applying an asymmetric waveform toat least one of the other electrode and the second segmented electrodesegment, whereby during use the second segmented electrode segment ismaintained at a different potential relative to the first and thirdsegmented electrode segments, such that a potential gradient is formedin a direction along the length of the segmented electrode assembly.

In accordance with the invention there is provided another apparatus forseparating ions, comprising: an analyzer region defined by a spacebetween first and second electrodes, the first electrode having a lengthand at least a quadratic ruled outer surface, the second electrodeoverlapping the first electrode along a first portion of the lengththereof and having at least a quadratic ruled inner surface opposite tothe at least a quadratic ruled outer surface of the first electrode; atleast a first contact on at least one of the first and second electrodesfor receiving a first direct current voltage, and for applying anasymmetric waveform voltage to the at least one of the first and secondelectrodes; a third electrode aligned with the second electrode fordefining a space between the third electrode and the first electrode,the third electrode overlapping the first electrode along a secondportion of the length thereof adjacent to the first portion of thelength thereof and other than overlapping with the first portion of thelength thereof; and, a second contact on the third electrode for atleast one of receiving a second direct current voltage and applying theasymmetric waveform voltage to the third electrode, whereby during usethe third electrode is maintained at a different potential relative tothe second electrode, such that a potential gradient is formed in adirection along the length of the first electrode.

In accordance with the invention there is provided yet another apparatusfor separating ions, comprising: an inner electrode having a length andan outer surface that is curved in a direction transverse to its lengthand an outer electrode having a length, a channel through a portionthereof, and a curved inner surface, a portion of the length of theouter electrode overlapping a portion of the length of the innerelectrode and forming an analyzer region therebetween, the outerelectrode being approximately coaxially aligned with the innerelectrode, the outer electrode including an ion inlet orifice and an ionoutlet orifice defined one each within facing surface portions withinthe length of the outer electrode overlapping a portion of the length ofthe inner electrode, at least one of the inner and outer electrodescomprising a segmented electrode comprised of an electrode segmentassembly, the electrode segment assembly including a plurality ofelectrode segments extending approximately a length coinciding with thelength of the at least one of the inner and outer electrode, a surfaceof each of the electrode segments being opposite a surface of the otherone of the inner and outer electrode; at least a first contact on one ofthe other one of the inner and outer electrode and a first segment ofthe segmented electrode for receiving a first direct current voltage,and for applying an asymmetric waveform to the one of the other one ofthe inner and outer electrode and the first segment of the segmentedelectrode; and, at least a second contact on a second segment of thesegmented electrode for receiving a second direct current voltage, so asto form a potential gradient in a direction along the lengths of theinner and outer electrodes.

In accordance with another aspect of the invention there is provided amethod for separating ions, comprising the steps of: providing ananalyzer region defined by a space between first and second spaced apartelectrodes; introducing ions into the analyzer region; providing a flowof a carrier gas through the analyzer region for directing the ionsalong a first direction within the analyzer region; providing a firstelectric field component within the analyzer region resulting from theapplication of an asymmetric waveform voltage and a direct currentcompensation voltage to at least one of the first and second electrodes,for directing the ions along a second direction within the analyzerregion that is approximately perpendicular to the first direction; and,providing a second electric field component within the analyzer regionfor directing the ions along a third direction within the analyzerregion, the third direction being approximately perpendicular to eachone of the first direction and the second direction.

In accordance with the other aspect of the invention there is provided amethod for separating ions, comprising the steps of: providing ananalyzer region defined by a space between inner and outer electrodeshaving at least a quadratic ruled surface, the inner electrode having alength, the outer electrode overlapping the inner electrode along apotion of the length thereof, transporting ions along an average ionflow path within the analyzer region, the average ion flow pathextending in a first direction approximately transverse to the length ofthe inner electrode and absent a substantially linear portion, theaverage ion flow path extending between an ion inlet orifice of theanalyzer region and an ion outlet orifice of the analyzer region;providing a radial electric field component within the analyzer regionresulting from the application of an asymmetric waveform voltage and adirect current compensation voltage to at least one of the inner andouter electrodes for effecting a separation of the ions; and, providinga longitudinal electric field component within the analyzer region fordirecting ions within the analyzer region in a second direction alongthe length of the inner electrode, the second direction beingapproximately perpendicular to the first direction.

In accordance with the invention there is provided yet another apparatusfor separating ions, comprising: an inner electrode and an outerelectrode arranged in an overlapping coaxial arrangement so as to forman analyzer region therebetween; at least a first contact on at leastone of the inner electrode and the outer electrode for providing a firstdirect current voltage difference between the inner electrode and theouter electrode, and for applying an asymmetric waveform to at least oneof the inner electrode and the outer electrode; an ion inlet orificewithin a first surface portion of the outer electrode for introducingions into the analyzer region and an ion outlet orifice within a secondsurface portion of the outer electrode for extracting ions from theanalyzer region, the first surface portion approximately facing thesecond surface portion; wherein at least one of the inner electrode andthe outer electrode comprises a segmented electrode comprising a firstelectrode segment having at least a second contact for providing asecond direct current voltage potential difference between the firstelectrode segment and the other one of the inner electrode and the outerelectrode.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will now be described inconjunction with the following drawings, in which similar referencenumbers designate similar items:

FIG. 1 a is a simplified cross sectional view of a cylindricalside-to-side FAIMS device according to the prior art;

FIG. 1 b is a side elevational view of the cylindrical side-to-sideFAIMS device shown in FIG. 1 a;

FIG. 2 is a side cross-sectional view of a domed-FAIMS device;

FIG. 3 is a simplified side view of a cylindrical side-to-side FAIMSdevice;

FIG. 4 is a simplified side view of a segmented side-to-side FAIMSdevice;

FIG. 5 is an enlarged side view of a portion of the segmentedside-to-side FAIMS device of FIG. 4;

FIG. 6 is a top view of the segmented side-to-side FAIMS device of FIG.4;

FIG. 7 a is a top view of a side-to-side FAIMS device, in which an innerelectrode has elliptic cylindrical shape;

FIG. 7 b is a top view of another side-to-side FAIMS device, in which aninner electrode has an elliptic cylindrical shape;

FIG. 7 c is a top view of a side-to-side FAIMS device, in which an innerelectrode has an elliptic hyperbolical shape;

FIG. 8 a, is a longitudinal cross sectional view through a side-to-sideFAIMS device having a segmented inner electrode;

FIG. 8 b, is a longitudinal cross sectional view through a side-to-sideFAIMS device having a segmented inner electrode, and a segmented outerelectrode;

FIG. 8 c is a longitudinal cross sectional view through a side-to-sideFAIMS device having a segmented outer electrode including five sections,and,

FIG. 9 is a simplified flow diagram for a method of longitudinallyconfining ions within a side-to-side FAIMS device.

DETAILED DESCRIPTION OF THE INVENTION

The following description is presented to enable a person skilled in theart to make and use the invention, and is provided in the context of aparticular application and its requirements. Various modifications tothe disclosed embodiments will be readily apparent to those skilled inthe art, and the general principles defined herein may be applied toother embodiments and applications without departing from the spirit andthe scope of the invention. Thus, the present invention is not intendedto be limited to the embodiments disclosed, but is to be accorded thewidest scope consistent with the principles and features disclosedherein.

Referring to FIG. 1 a, shown is a simplified cross sectional view of acylindrical side-to-side FAIMS according to the prior art. Thecylindrical side-to-side FAIMS device, shown generally at 10, includesinner and outer cylindrical electrodes 12 and 14, respectively, whichare supported by an electrically insulating material (not shown) in anoverlapping, spaced-apart arrangement. The generally annular spacebetween the inner electrode 12 and the outer electrode 14 defines aFAIMS analyzer region 16. The analyzer region 16 is of approximatelyuniform width and extends around the circumference of the innerelectrode 12. An ion inlet orifice 18 is provided through the outerelectrode 14 for introducing ions from an ion source into the analyzerregion 16. For example, the ion source is in the form of an electrosprayionization ion source including a liquid delivery capillary 24, afine-tipped electrospray needle 22 that is held at high voltage (powersupply not shown), and a curtain plate 26 serving as a counter-electrodefor the electrospray needle 22. Of course, any other suitable ionizationsource is optionally used in place of the electrospray ionization ionsource. A flow of a carrier gas, which is represented in the figure by aseries of closed-headed arrows, is provided within the analyzer region16 to carry the ions around the inner electrode 12 and toward an ionoutlet orifice 20. An orifice 25 within the curtain plate electrode 26allows for the flow of a portion of the carrier gas in a direction thatis counter-current to the direction in which the ions are traveling nearthe ion inlet orifice 18, so as to desolvate the ions before they areintroduced into the analyzer region 16. The inner electrode 12 is inelectrical communication with a power supply 28 that during use iscapable of applying a high voltage asymmetric waveform voltage (DV) anda low voltage dc compensation voltage (CV) to the inner FAIMS electrode12.

Still referring to FIG. 1 a, ions are produced in the gas phase at thefine-tipped electrospray needle 22 from a suitable sample containing aspecies of interest. Typically, a mixture including a plurality ofdifferent ion types is produced when the sample is ionized. Thepotential gradient accelerates the ions of the mixture away from theelectrospray needle 22, toward the curtain plate electrode 26. A portionof the ions pass through the orifice 25 in the curtain plate electrode26, become entrained in the carrier gas flow and are carried into theFAIMS analyzer region 16. Once inside the FAIMS analyzer region 16, theions are carried through an electric field that is formed within theFAIMS analyzer region 16 by the application of the DV and the CV to theinner FAIMS electrode 12. Ion separation occurs within the FAIMSanalyzer region 16 on the basis of the high field mobility properties ofthe ions. Those ions of the mixture that have a stable trajectory for aparticular combination of DV and CV are selectively transmitted throughthe FAIMS analyzer region 16, whilst other ions of the mixture collidewith an electrode surface and are lost. The selectively transmitted ionsare extracted from the analyzer region 16 via ion outlet orifice 20 andare typically subjected to one of detection and further analysis.

Referring now to FIG. 1 b, shown is a simplified side elevational viewof the cylindrical side-to-side FAIMS of FIG. 1 a. Elements labeled withthe same numerals have the same function as those illustrated in FIG. 1a. The dotted line extending between ion inlet orifice 18 and ion outletorifice 20 represents one possible average ion flow path around theinner electrode 12. An average ion flow path is defined as the nettrajectory of an ion as a result of a carrier gas flow through theanalyzer region, although the individual ion also experiences anoscillatory motion between the electrodes as a result of the appliedasymmetric waveform voltage. In particular, the dotted line representsone of two shortest average ion flow paths through the analyzer region16, one shortest average ion flow path extending in each directionaround the inner electrode 12. Of course, when many like-charged ionsare present within the analyzer region, ion-ion repulsion forces tend tocause the ions to spread out slightly along the length of the innerelectrode 12. Accordingly, some selectively transmitted ions migrateinto portions of the analyzer region where the gas flow rate is low orstagnant, making their extraction from the analyzer region difficult.

Referring now to FIG. 2, shown is a side cross-sectional view of adomed-FAIMS device. The domed-FAIMS, shown generally at 30, includesinner and outer cylindrical electrodes 32 and 34, respectively, in aspaced apart arrangement defining an analyzer region 36 therebetween forseparating ions introduced via an ion inlet orifice 38. In particular,FIG. 2 illustrates the manner in which the electric fields and the gasflows interact with the ions within the analyzer region 36. The electricfields restrict the movement of the ions between the electrodes, in aradial direction defined with respect to the center of the innerelectrode 32 being taken as the axis of rotation, whereas the carriergas flows along the length of the analyzer region 36 to transport theions from an ion inlet orifice 38 toward an ion outlet orifice 40. Atthe region in front of a hemispherical tip 42 of the inner electrode,the ions are directed toward the ion outlet orifice 40 as a narrow beamthat is suitable for introduction into a not illustrated massspectrometer.

Referring now to FIG. 3, shown is a simplified side view of acylindrical side-to-side FAIMS device. With the side-to-side version ofFAIMS, shown generally at 50, the electric fields again work to keep theions between an inner and an outer electrode 52 and 54, respectively.The electric fields restrict the movement of the ions between theelectrodes, in a radial direction defined with respect to the center ofthe inner electrode 52 being taken as the axis of rotation; however, thecarrier gas fans out between an ion inlet orifice 56 and an ion outletorifice 58, so as to transmit ions along a plurality of average ion flowpaths therebetween. Only a portion of the ions introduced through theion inlet orifice 56 follow a shortest ion average flow path to the ionoutlet orifice 58. Accordingly, ion losses due to diffusion and spacecharge repulsion are increased in a side-to-side FAIMS device 50relative to a dome FAIMS device 30. Of course, ion losses due todiffusion and space charge repulsion are expected to be the mostpronounced for ions with higher mobility values, such as for instancechloride ions, and least noticeable for ions with lower mobility values,such as for instance protein ions.

Referring now to FIG. 4, shown is a simplified side view of a segmentedside-to-side FAIMS device according to the instant invention. FIG. 4shows only the conductive components of this device; for instance, theinsulating PEEK material and some of the components of ionization sourcehave been omitted for the sake of clarity. The segmented side-to-sideFAIMS, shown generally at 60, includes a cylindrical inner electrode 62as well as first, second and third outer electrode segments 64, 66 and68, respectively, which are supported by not illustrated electricallyinsulating material in an overlapping, spaced-apart arrangement with theinner electrode 62. The generally annular space between the innerelectrode 62 and the second outer electrode segment 66 defines a FAIMSanalyzer region 70. The analyzer region 70 is of approximately uniformwidth and extends around the circumference of the inner electrode 62between an ion inlet orifice 72 and an ion outlet orifice 74. The ioninlet oiifice 72 is provided through the second outer electrode segment66 for introducing ions from an ion source into the analyzer region 70.For example, the ion source is in the form of an electrospray ionizationion source including a liquid delivery capillary 76, a fine-tippedelectrospray needle 78 that is held at high voltage (power supply notshown), and a curtain plate 80 serving as a counter-electrode for theelectrospray needle 78. Of course, any other suitable ionization sourceis used optionally in place of the electrospray ionization ion source. Aflow of a carrier gas, which is represented in the figure by a series ofclosed-headed arrows, is provided within the analyzer region 70 to carrythe ions around the inner electrode 62 and toward the ion outlet orifice74. An orifice 81 within the curtain plate electrode 80 allows for theflow of a portion of the carrier gas in a direction that iscounter-current to the direction in which the ions are traveling nearthe ion inlet orifice 72, so as to desolvate the ions before they areintroduced into the analyzer region 70. The inner electrode 62 is inelectrical communication with a power supply 82 that during use iscapable of applying a high voltage asymmetric waveform voltage (DV) anda low voltage dc compensation voltage (CV) to the inner FAIMS electrode62.

Referring still to FIG. 4, the inner electrode 62 remains unchanged fromprior art side-to-side FAIMS devices, whilst the outer FAIMS electrodeis provided as an assembly of three separate outer electrode segments64, 66 and 68, all of which are electrically isolated one from another.A power supply 90 is provided in electrical communication with the firstouter electrode segment 64 that during use is capable of applying a dcoffset potential to the first outer electrode segment 64. For instance,the power supply 90 applies a dc potential in the range of 0–50 voltsrelative to the second outer electrode segment 66. Most preferably, thepower supply 90 applies a dc potential in the range of 0–15 voltsrelative to the second outer electrode segment 66. Of course, thepolarity of the voltage may be either positive or negative, and isdetermined in dependence upon the polarity of the ions being selectivelytransmitted within the analyzer region 70. A power supply 88 is alsoshown in electrical communication with the third outer electrode segment68 for applying a dc offset potential to the third outer electrodesegment 68. Optionally, a single power supply in electricalcommunication with the first and third outer electrode segments replacesthe two separate power supplies 88 and 90. Of course, when a singlepower supply is used, it is not possible to apply different dc offsetpotentials individually to the first and third outer electrode segments.

Of course, a separate power supply (not shown) is also provided inelectrical communication with the second outer electrode segment 66 formaintaining the second outer electrode segment 66 at a desired dcvoltage.

Referring still to FIG. 4, the segmented side-to-side FAIMS device 60 issealed gas tight against an orifice plate 84 of a not illustrated massspectrometer. Additionally, the FAIMS device 60 is assembled such thatthere is no gas leaks, in other words, the carrier gas and ions are onlyable to enter or exit the space between the inner electrode 62 and theouter electrode segments 64, 66 and 68 through one of the ion inletorifice 72 and the ion outlet orifice 74, respectively, both of whichare located in the second outer electrode segment 66. To this end, aninsulating material (not shown) is disposed between opposing ends ofouter electrode segments 64 and 66, and between opposing ends of outerelectrode segments 66 and 68. The insulating material is selected tomaintain electrical isolation of the various conductive surfaces shownin FIG. 4. PEEK is one suitable material for use as the insulatingmaterial.

Referring now to FIG. 5, shown is an enlarged side view of a portion ofthe segmented side-to-side FAIMS device 60 shown at FIG. 4. Inparticular, FIG. 5 shows an enlarged view of the region in which theorifice plate 84, the first outer electrode segment 64, and the secondouter electrode segment 66 are combined. Insulating material 86 is shownin the enlarged view, and is represented by the dotted lines between thevarious conductive surfaces. The insulating material 86 preferablysatisfies a number of criteria. First, the insulating material 86maintains electrical isolation of the various conductive surfaces shownin FIG. 4, such as for example between adjacent outer electrodesegments. Preferably, the insulating material 86 is thin, such that theelectric fields in the analyzer region are not adversely affected by acharge build-up on the insulating material 86, in particular between theouter electrode segments. Additionally, the insulating material 86 thatis disposed between the first outer electrode segment 64 and the secondouter electrode segment 66, and between the second outer electrodesegment 66 and the third outer electrode segment 68, should not extendinside of the inner wall of any of the outer electrode segments. Mostpreferably, the insulating material is recessed within the space that isformed between adjacent outer electrode segments. Of course, theinsulating material 86 preferably forms a gas-tight seal between theconductive surfaces so as to prevent gas from escaping between theconductive surfaces, which would lead to additional ion losses. Forexample, carrier gas escaping through other than the ion outlet orifice74 reduces the total flow through the analyzer region 70 and into thenot illustrated mass spectrometer, which results in increased iontransit time through the device 60 and increased ion losses,respectively. Furthermore, a flow of carrier gas escaping from betweenthe electrodes may carry ions toward the electrodes, leading to a highernumber of ions being lost as a result of a collision with an electrodesurface, thereby further decreasing ion transmission efficiency.Alternatively, the insulating material 86 does not form a gas-tight sealbetween the conductive surfaces, and the effects of the gas escapingfrom between the conductive surfaces are compensated for. For instance,a supplemental flow of carrier gas is introduced into the analyzerregion 70 through the spaces between the conductive surfaces, so as toreduce ion loss.

Referring now to FIG. 6, shown is a top view of the segmentedside-to-side FAIMS device shown at FIG. 4. FIG. 6 shows an example ofone preferred configuration of the insulating material 86. Inparticular, the insulating material between adjacent outer electrodesegments is provided in the form of a ring of insulating material thatis thin both in the radial and longitudinal directions. As shown in FIG.6, the ring of insulating material is thin compared to the wallthickness of the outer electrode segments. Of course, any suitablematerial may be used as the insulating material 86, such as for exampleone of PEEK and Teflon™ tape. Preferably, Teflon™ tape is used inapplications where the voltage difference is small, such as for exampleless than 10V. In particular, Teflon™ tape may be used between theorifice plate 84 and each one of the outer electrode segments.

During use, ions are produced in the gas phase for introduction into thesegmented side-to-side FAIMS device 60 from a suitable sample containinga species of interest. Typically, a mixture including a plurality ofdifferent ion types is produced when the sample is ionized. Thepotential gradient accelerates the ions of the mixture away from theelectrospray needle 78, toward the curtain plate electrode 80. A portionof the ions pass through the orifice 81 in the curtain plate electrode80, become entrained in the carrier gas flow and are carried into theFAIMS analyzer region 70. Once inside the FAIMS analyzer region 70, theions are carried through an electric field that is formed within theFAIMS analyzer region 70 by the application of the DV and the CV to theinner FAIMS electrode 62. Ion separation occurs within the FAIMSanalyzer region 70 on the basis of the high field mobility properties ofthe ions. Those ions of the mixture that have a stable trajectory for aparticular combination of DV and CV are selectively transmitted throughthe FAIMS analyzer region 70, whilst other ions of the mixture collidewith an electrode surface and are lost. The selectively transmitted ionsare extracted from the analyzer region 70 via ion outlet orifice 74 andare typically subjected to one of detection and further analysis.

The ions with the appropriate properties for transmission for a givenset of applied experimental conditions are confined in a radialdirection between the second outer electrode segment 66 and the innerelectrode 62. When all of the outer electrode segments 64, 66, 68 are atthe same applied dc voltage, however, diffusion and space chargerepulsion act to displace ions away from regions of higher ion density,such as for example along an axis intersecting the ion inlet orifice 72and ion outlet 74. Referring again to FIG. 4, an ion that is located atposition A is within the region of higher ion density, an ion that islocated at position B is within a region of intermediate ion density,and an ion that is located at position C is within a region ofrelatively low ion density. For the purposes of the followingdiscussion, each one of the ions that are located at positions A, B andC are assumed to be positive ions with identical ion mobilityproperties.

By placing a positive dc voltage on the first outer electrode segment 64and the third outer electrode segment 68 relative to the second outerelectrode segment 66, for example, +5 V, an electric field isestablished along the length of the electrodes of the segmentedside-to-side device 60. This electric field acts to minimize losses dueto diffusion and space charge repulsion, and therefore increases iontransmission efficiency. The effect of the electric fields in thelongitudinal direction on the ion that is located at position B is topush the ion back toward location A. Of course, in addition to beingpushed back toward location A the ion also experiences a push toward theinner electrode.

In the case of an ion that is located at position C, changing the dcvoltage on the first outer electrode segment 64, relative to the secondouter electrode segment 66, causes the CV between the first outerelectrode segment 64 and the inner electrode 62 to be different than theCV between the second outer electrode segment 66 and the inner electrode62. Consequently, instead of having the desired effect of pushing theion back toward location A to prevent ion loss, the application of thedc voltage causes the ion that was focused between the first outerelectrode segment 64 and the inner electrode 62, when no dc voltage wasapplied, to collide with the inner electrode. Accordingly, the voltagesthat are applied to the first and third outer electrode segments 64, 68,respectively, must be optimized so that any fringing fields resultingfrom these voltages do not significantly disrupt the focusing fieldsthat are required for efficient ion transmission within the analyzerregion 70 between the second outer electrode segment 66 and the innerelectrode 62.

Referring again to FIG. 4, each one of the outer electrode segments isshown as being substantially the same size, in terms of the length axis.Optionally, the individual outer electrode segments are of differentlengths. For instance, the first outer electrode segment 64 and thethird outer electrode segment 68 are of a same length that is longerthan the length of the second outer electrode segment 66. The length ofeach individual outer electrode segment, as well as the dc offsetvoltages applied to the first outer electrode segment 64 and to thethird outer electrode segment 68, can be optimized experimentally.Optionally, segment lengths and dc offset voltages are predetermined forparticular applications. Further optionally, a number of outer electrodesegments other than three is used.

The concepts applied in the construction of a segmented side-to-sideFAIMS device have been illustrated for one particular embodiment of asegmented side-to-side FAIMS device, namely a side-to-side FAIMS devicecomprising a cylindrical inner electrode and a threefold-segmented outerelectrode. A person of skill in the art realises that the ideasillustrated above are to be generalized to include various shapes ofinner and outer electrodes, as well as various types of electrodesegmentation patterns.

Generally, the shape of the inner and outer electrodes is chosen suchthat the surfaces of the inner and outer electrodes, which constitutethe boundaries of an analyzer region, include but are not limited toquadratic ruled surfaces and surfaces of revolution. Examples forquadratic ruled surfaces are the cylinder, the elliptic cylinder, thehyperboloid, and the elliptic hyperboloid. Alternatively, the surfacesof the inner and outer electrodes enclosing the analyzer region areexpressed as superposition of quadratic ruled surfaces. Furtheralternatively, the surfaces have a crosssection in a plane parallel tothe average ion flow path that is curved in perimeter. Preferably theperimeter has equal distances along each of two paths between the ioninlet orifice and the ion outlet orifice. Alternatively, the perimeterincludes at least one curved line symmetric about a line formed betweenthe ion inlet orifice and the ion outlet orifice. Preferably, at mosttwo curved lines form the perimeter. Further preferably, a curvedsurface of an electrode is curved in a direction transverse to a lengthof the electrode.

The surface of the inner electrode forming a boundary of an analyzerregion is also referred to as the outer surface of the inner electrode,and the surface of the outer electrode forming a boundary of an analyzerregion is also referred to as the inner surface of the outer electrode.The outer surface of the inner electrode as well as the inner surface ofthe outer electrode are adjacent to each other, and opposing points onthe inner and outer surface are all spaced apart by approximately aconstant distance, or by variable distances.

In FIGS. 7 a–c, shown are top views of three different electrodeconfigurations of a segmented side-to-side FAIMS device according toembodiments of the instant invention. Referring to FIG. 7 a, an innerelectrode 7162 has an elliptic cylindrical shape, and is orientedrelative to an outer electrode 7166 such that the major axis of theelliptical cross section is substantially perpendicular to the lineconnecting an ion inlet orifice 7172 and an ion outlet orifice 7174,both disposed in the outer electrode 7166. The outer electrode 7166 hasan inner surface of elliptic cylindrical shape, so as to maintain anapproximately uniform spacing between the electrode segments enclosingthe analyzer region.

Referring now to FIG. 7 b, an inner electrode 7262 has an ellipticcylindrical shape, and is oriented relative to an outer electrode 7266such that the major axis of the elliptical cross section issubstantially parallel to the line connecting an ion inlet orifice 7272and an ion outlet orifice 7274, both disposed in the outer electrode7266. The outer electrode 7266 has an inner surface that is of ellipticcylindrical shape, so as to maintain an approximately uniform spacingbetween the electrode segments enclosing the analyzer region.

Referring to FIG. 7 c, an inner electrode 7362 has an elliptichyperbolical shape, and is oriented relative to an outer electrode 7366such that line connecting the two foci of an hyperbolic cross section issubstantially perpendicular to the line connecting an ion inlet orifice7372 and an ion outlet orifice 7374, both disposed in the outerelectrode 7366. The outer electrode 7366 has an inner surface that is ofelliptic hyperbolical shape, so as to maintain an approximately uniformspacing between the electrode segments enclosing the analyzer region.

Alternatively, an inner electrode and an outer electrode have differentshapes. The outer surface of an inner electrode for example has anelliptic cylindrical shape, whereas the inner surface of an outerelectrode has a regular cylindrical shape. The varying distance betweeninner and outer electrode is possibly favourably utilized for analyzingan ion beam containing a plurality of distinct ions. A person of skillin the art easily envisions different electrode configurations, for allof which the surfaces enclosing an analyzer region are describable assuperposition of quadratic ruled surfaces.

In the examples given above, electrode segmentation has been describedfor cases in which an outer electrode is segmented into three separateouter electrode segments. An intermediate segment of the outerelectrode, and the inner electrode constitute an analyzer region of aside-to-side FAIMS device, whereas outer segments of the outer electrodefocus an ion beam, and provide functionality of an ion lens.Alternatively, this functionality is also achieved when the innerelectrode is segmented. Referring now to FIG. 8 a, shown is alongitudinal cross sectional view through a segmented side-to-side FAIMSdevice. Three inner electrode segments 8164, 8166 and 8168 are provided,as well as an outer electrode 8170. The inner electrode segment 8166 andthe corresponding part of the outer electrode 8170 constitute theanalyzer region of the FAIMS device, whereas the inner electrodesegments 8164 and 8168 provide similar functionality as described forthe electrode segments 64 and 68 (i.e. during use a high voltageasymmetric waveform voltage and a low voltage dc compensation voltage isapplied between the inner electrode segment 8166 and the outer electrode8170. and a high voltage asymmetric waveform voltage and a low voltagedc compensation voltage is applied between each inner electrode segment8164 or 8168 and the outer electrode 8170).

Referring now to FIG. 8 b, shown is a longitudinal cross sectional viewthrough a side-to-side FAIMS device, having segmented inner as well asouter electrodes. The inner electrode is segmented into a first innerelectrode segment 8164, a second inner electrode segment 8166, and athird inner electrode segment 8168. The outer electrode is segmentedinto a first outer electrode segment 8174, a second outer electrodesegment 8176, and a third outer electrode segment 8178. The innerelectrode segment 8166 and the outer electrode segment 8176 constitutethe analyzer region of the side-to-side FAIMS device, whereas theremaining electrodes function as to enhance, guide, and focus an ionbeam. An ion inlet orifice 8192 and an ion outlet orifice 8198 are bothdisposed in the outer electrode segment 8176. In the discussion thatfollows, the first inner electrode segment 8164 and the first outerelectrode segment 8174 comprise a first electrode segment pair. Secondand third electrode segment pairs are similarly defined in terms of theremaining inner and outer electrode segments.

When both the inner electrode and the outer electrode are segmented, itis preferable to separately apply a same DV between each inner/outerelectrode segment pair. The potentials that are applied to the inner andouter electrode segments of each one of the first and third inner/outerelectrode segment pairs are floated relative to the potentials that areapplied to the second inner/outer electrode pair, while maintaining asame CV value between each of the first, second, and third inner/outerelectrode segment pairs. Accordingly, a same combination of CV and DVexists for selectively transmitting an ion of interest between eachinner/outer electrode segment pair. Since the dc potentials applied toeach one of the first and third electrode segment pairs are “floated”relative to the second electrode segment pair, ions being transmittedbetween the second inner/outer electrode segment pair from the ion inletorifice to the ion outlet orifice will “see” a potential gradient nearan interface between the second inner/outer electrode segment pair andeach one of the first and third inner/outer electrode segment pairs.This potential gradient tends to prevent the ions from spreading outinto a space between either one of the first and third inner/outerelectrode segment pairs, even though appropriate conditions fortransmitting the ions exist within the space.

Referring to FIG. 8 c, another embodiment of the invention is shownwherein an outer electrode is segmented circumferentially into fiveperipheral sections 8871, 8872, 8873, 8874, and 8875, the peripheralsections enclosing an inner electrode 8870. Sections 8872 and 8874 serveas analyzer sections, whereas sections 8871, 8873, and 8875 serve asboundary sections. It is of advantage that section 8873 serves asboundary section not only for one, but for two analyzer sections. Ofcourse, other numbers of electrodes are also supported. The sectionsallow for application of different bias voltages at different locationswithin the peripheral sections. Of course, other applications of thesegmented electrodes are also supported. Optionally, it is the innerelectrode that is segmented in a similar fashion.

Of course, the segmentation of the FAIMS analyser in accordance with theinvention is also useful for providing non-overlapping analyser regionsthat are isolated one from another.

Referring now to FIG. 9, shown is a simplified flow diagram for a methodof improving ion transmission efficiency of a side-to-side FAIMS deviceby longitudinally confining the ions during separation. At step 100, aflow of a carrier gas is provided through an analyzer region of asegmented side-to-side FAIMS device, such as device 60 shown at FIG. 4.At step 102 ions are introduced into the analyzer region, for instancethe ions are produced at an ion source, entrained into the flow of thecarrier gas and subsequently carried into the analyzer region with thecarrier gas. At step 104, a first electric field component is providedwithin the analyzer region, the first electric field component resultingfrom the application of an asymmetric waveform voltage and a directcurrent compensation voltage to at least one of the inner electrode 62and the first, second, and third outer electrode segments 64, 66 and 68,respectively. The first electric field component is a radial electricfield component for focusing the selectively transmitted ions as theymove through the analyzer region. At step 106, a second electric fieldcomponent is provided within the analyzer region. For instance, apositive dc potential is applied to each one of the first outerelectrode segment 64 and the third outer electrode segment 68, to directpositively charged ions along a direction that is counter to thedirection in which the ions are caused to drift as a result of ion-ionrepulsion forces. The second electric field component is a longitudinalelectric field that directs the ions along a direction that isapproximately normal to the net ion trajectory through the analyzerregion as a result of the carrier gas flow. The ions are directed by thesecond electric field component along a direction that is approximatelyperpendicular to the direction of radial focusing as a result of theapplied CV and DV. Of course, the step of providing the second electricfield component optionally includes a step of varying the positive dcpotential that is applied to each one of the first outer electrodesegment 64 and the third outer electrode segment 68, to obtain a maximumion transmission efficiency. Alternatively, the positive dc potential isset to a predetermined value. Of course, a positive dc potential isapplied in the case of positively charged selectively transmitted ions,whereas a negative dc potential is applied in the case of negativelycharged selectively transmitted ions.

The electric fields for selectively transmitting ions through a FAIMSanalyzer region are established optionally by applying an asymmetricwaveform voltage and a dc compensation voltage to an outer electrode ofthe FAIMS, by applying an asymmetric waveform voltage and a dccompensation voltage to an inner electrode of the FAIMS, or by applyingone of an asymmetric waveform voltage and a dc compensation voltage toone of an inner electrode and an outer electrode of the FAIMS and theother one of the asymmetric waveform voltage and the dc compensationvoltage to the other one of the inner electrode and the outer electrodeof the FAIMS. In the event that an asymmetric waveform voltage is beingapplied to a segmented electrode comprising a plurality of electricallyisolated electrode segments, then it is necessary to separately apply asame asymmetric waveform voltage to each one of the plurality ofelectrode segments.

Numerous other embodiments may be envisaged without departing from thespirit and scope of the invention.

1. An apparatus for separating ions, comprising: an analyzer regiondefined by a space between first and second electrodes, the firstelectrode having a length and at least a quadratic ruled outer surface,the second electrode overlapping the first electrode along a firstportion of the length thereof and having at least a quadratic ruledinner surface opposite to the at least a quadratic ruled outer surfaceof the first electrode; at least a first contact on at least one of thefirst and second electrodes for receiving a first direct currentvoltage, and for applying an asymmetric waveform voltage to the at leastone of the first and second electrodes; a third electrode aligned withthe second electrode for defining a space between the third electrodeand the first electrode, the third electrode overlapping the firstelectrode along a second portion of the length thereof adjacent to thefirst portion of the length thereof and other than overlapping with thefirst portion of the length thereof; and, a second contact on the thirdelectrode for at least one of receiving a second direct current voltageand applying the asymmetric waveform voltage to the third electrode,whereby during use the third electrode is maintained at a differentpotential relative to the second electrode, such that a potentialgradient is formed in a direction along the length of the firstelectrode.
 2. An apparatus according to claim 1, comprising an ion inletorifice disposed within a first surface portion of the second electrodefor introducing ions and a flow of a carrier gas into the analyzerregion and an ion outlet orifice disposed within a second surfaceportion of the second electrode opposing the first surface portion forextracting ions from the analyzer region.
 3. An apparatus according toclaim 2, wherein during use the ions are selectively transmitted withinthe analyzer region between the ion inlet orifice and the ion outletorifice along an average ion flow path absent a substantially linearportion, and wherein the ions are affected by the potential gradientsuch that ions within the average ion flow path are focused in adirection along the length of the first electrode and transverse to theaverage ion flow path.
 4. An apparatus according to claim 2, comprisinga control circuit for providing the potential difference between thesecond electrode and the third electrode of between 0 volts and 50volts.
 5. An apparatus according to claim 2, comprising a controlcircuit for providing the potential difference between the secondelectrode and the third electrode of between 0 volts and 2 volts.
 6. Anapparatus for separating ions, comprising: an inner electrode having alength and an outer surface that is curved in a direction transverse toits length, and an outer electrode having a length, a channel through aportion thereof, and a curved inner surface, a portion of the length ofthe outer electrode overlapping a portion of the length of the innerelectrode and forming an analyzer region therebetween, the outerelectrode being approximately coaxially aligned with the innerelectrode, the outer electrode including an ion inlet orifice and an ionoutlet orifice defined one each within facing surface portions withinthe length of the outer electrode overlapping a portion of the length ofthe inner electrode, at least one of the inner and outer electrodescomprising a segmented electrode comprised of an electrode segmentassembly, the electrode segment assembly including a plurality ofelectrode segments extending approximately a length coinciding with thelength of the at least one of the inner and outer electrode, a surfaceof each of the electrode segments being opposite a surface of the otherone of the inner and outer electrode; at least a first contact on one ofthe other one of the inner and outer electrode and a first segment ofthe segmented electrode for receiving a first direct current voltage,and for applying an asymmetric waveform to the one of the other one ofthe inner and outer electrode and the first segment of the segmentedelectrode; and, at least a second contact on a second segment of thesegmented electrode for receiving a second direct current voltage, so asto form a potential gradient in a direction along the lengths of theinner and outer electrodes.
 7. A method for separating ions, comprisingthe steps of: providing an analyzer region defined by a space betweenfirst and second spaced apart electrodes; introducing ions into theanalyzer region; providing a flow of a carrier gas through the analyzerregion for directing the ions along a first direction within theanalyzer region; providing a first electric field component within theanalyzer region resulting from the application of an asymmetric waveformvoltage and a direct current compensation voltage to at least one of thefirst and second electrodes, for directing the ions along a seconddirection within the analyzer region that is approximately perpendicularto the first direction; and, providing a second electric field componentwithin the analyzer region for directing the ions along a thirddirection within the analyzer region, the third direction beingapproximately perpendicular to each one of the first direction and thesecond direction.
 8. A method according to claim 7, including the stepof applying a direct current voltage to third and fourth electrodes,such that a potential difference between the third electrode and thefirst electrode and a potential difference between the fourth electrodeand the first electrode is different than a potential difference betweenthe second electrode and the first electrode.
 9. A method according toclaim 7, including the step of varying the provided second electricfield component so as to increase the ion transmission efficiencythrough the analyzer region.
 10. A method for separating ions,comprising the steps of: providing an analyzer region defined by a spacebetween inner and outer electrodes having at least a quadratic ruledsurface, the inner electrode having a length, the outer electrodeoverlapping the inner electrode along a potion of the length thereof;transporting ions along an average ion flow path within the analyzerregion, the average ion flow path extending in a first directionapproximately transverse to the length of the inner electrode and absenta substantially linear portion the average ion flow path extendingbetween an ion inlet orifice of the analyzer region and an ion outletorifice of the analyzer region; providing a radial electric fieldcomponent within the analyzer region resulting from the application ofan asymmetric waveform voltage and a direct current compensation voltageto at least one of the inner and outer electrodes for effecting aseparation of the ions; and, providing a longitudinal electric fieldcomponent within the analyzer region for directing ions within theanalyzer region in a second direction along the length of the innerelectrode, the second direction being approximately perpendicular to thefirst direction.
 11. An apparatus for separating ions, comprising: aninner electrode and an outer electrode arranged in an overlappingcoaxial arrangement so as to form an analyzer region therebetween; atleast a first contact on at least one of the inner electrode and theouter electrode for providing a first direct current voltage differencebetween the inner electrode and the outer electrode, and for applying anasymmetric waveform to at least one of the inner electrode and the outerelectrode; an ion inlet orifice within a first surface portion of theouter electrode for introducing ions into the analyzer region and an ionoutlet orifice within a second surface portion of the outer electrodefor extracting ions from the analyzer region, the first surface portionapproximately facing the second surface portion; wherein at least one ofthe inner electrode and the outer electrode comprises a segmentedelectrode comprising a first electrode segment having at least a secondcontact for providing a second direct current voltage potentialdifference between the first electrode segment and the other one of theinner electrode and the outer electrode.
 12. An apparatus according toclaim 11, comprising a second electrode segment, the first electrodesegment and the second electrode segments being disposed such that theion inlet orifice and the ion outlet orifice are between the firstelectrode segment and the second electrode segment, the second electrodesegment comprising at least a third contact for providing a third directcurrent voltage difference between the second electrode segment and theother one of the inner electrode and the outer electrode.
 13. Anapparatus according to claim 12, comprising a control circuit forproviding the second direct current voltage difference between the firstelectrode segment and the other one of the inner electrode and the outerelectrode, and for providing the third direct current voltage differencebetween the second electrode segment and the other one of the innerelectrode and the outer electrode, so as to establish potentialgradients within the analyzer region for directing ions through the ionoutlet orifice with an efficiency that is improved relative to anefficiency absent the potential gradients being established.
 14. Anapparatus according to claim 13, wherein during use the ions areselectively transmitted within the analyzer region between the ion inletorifice and the ion outlet orifice along an average ion flow path absenta substantially linear portion, and wherein the ions are affected by thepotential gradients such that ions within the average ion flow path arefocused in a direction transverse to the average ion flow path.
 15. Anapparatus according to claim 11, wherein the inner electrode comprisesthe segmented electrode.
 16. An apparatus according to claim 11, whereinthe outer electrode comprises the segmented electrode.
 17. An apparatusaccording to claim 16, wherein the inner electrode comprises a secondsegmented electrode.
 18. An apparatus according to claim 11, wherein theinner electrode includes an outer surface that follows a curve definedby a quadratic ruled function.
 19. An apparatus according to claim 18,wherein the quadratic ruled function defines a regular cylinder.
 20. Anapparatus according to claim 18, wherein the quadratic ruled functiondefines an elliptic cylinder.
 21. An apparatus according to claim 11,wherein the outer electrode includes an inner surface that follows acurve defined by a quadratic ruled function.
 22. An apparatus accordingto claim 21, wherein the quadratic ruled function defines a regularcylinder.
 23. An apparatus according to claim 21, wherein the quadraticruled function defines an elliptic cylinder.